Learn Russian: У ворот стояли двое строгих охранников.

You can actually use either! However, collective numerals like двое are very commonly used with masculine nouns that refer to male people, like охранников (guards). It emphasizes them as a collective unit rather than two separate individuals.

Why are the words строгих and охранников in the genitive plural?

Collective numerals in the nominative case, such as двое or трое, require the adjectives and nouns following them to take the genitive plural. This is why the masculine singular строгий охранник becomes строгих охранников.

What is the dictionary form of ворот, and why does it look like a shortened word here?

The dictionary form is ворота (gates), which is a word that only ever exists in the plural. The preposition у (at/by) requires the genitive case, and the genitive plural of ворота simply drops the final vowel, leaving just the zero-ending form ворот.

The verb стояли is plural. Does it have to be plural when using двое?

When a collective numeral like двое is the subject and refers to living people, the verb generally takes the plural form like стояли to emphasize that multiple individuals are actively doing the action.